When President Obama endorsed gay marriage earlier this month, the media grappled with two basic political questions: Was his personal “evolution” a case of a politician transparently following a national trend toward accepting same-sex unions (accelerated, perhaps, by his chatty No. 2), and would it hurt his re-election chances by alienating socially conservative voters like black churchgoers? Sure, there was a recognition that it marked a gratifying moment for gay-marriage advocates — as well as some grumbling about the President’s view that it remains a state issue, not a federal one. But by and large, there were few suggestions that one man, even the President, would shift public opinion on the issue or affect public policy. Based on a new Public Policy Polling survey out of Maryland, it seems this possibility was underestimated.

The poll, conducted after Obama came out, found a 12-point swing toward support for gay marriage in the state. That’s not insignificant on its own, but the demographics of the shift are more telling:

The movement over the last two months can be explained almost entirely by a major shift in opinion about same-sex marriage among black voters. Previously 56% said they would vote against the new law with only 39% planning to uphold it. Those numbers have now almost completely flipped, with 55% of African Americans planning to vote for the law and only 36% now opposed.

That’s a quantum leap in a very short period of time, and it comes after national groups like the NAACP followed Obama on the issue.

There are real policy implications here too. Maryland’s statehouse just legalized gay marriage in February, and its opponents are now efforting a repeal-by-referendum movement for the fall, driven in part by black pastors and their flocks. Across the U.S., gay marriage has an atrocious record at the ballot box (0 for 32), even in states that have approved gay marriage legislatively. So if Obama’s announcement caved in the anti-gay-marriage movement in Maryland, it might make all the difference.

When we look at this nationally, the picture is less clear. A President’s personal power of persuasion is a hard thing to quantify, though it’s reasonable to assume that Obama, the first black President, has a unique effect on black public opinion. While the electorate and particulars of Maryland’s situation are not present in other states, the polling nonetheless demonstrates that some people are following Obama’s personal change of heart, at least in greater numbers than I would have expected. And this has implications beyond legislatures and referendums.

The real battle for gay marriage is currently playing out in the courts, and so the week of Obama’s announcement, I asked conservative lawyer Ted Olson, who’s arguing the case against California’s gay-marriage ban in the Ninth Circuit Court of Appeals, whether it had a chance of changing minds in the judiciary. “It could have an effect, although it’s intangible and immeasurable,” Olson said. “The courts live in the same world that the rest of us live in. Judges are aware of what’s happening around them, and I think that they would be aware of what the President has said. The same thing is true of the President’s decision not to defend the Defense of Marriage Act, and I think that’s also true of changes in public opinion. I think that’s all part of the atmosphere in which decisions are made.”

Obama, whatever his motivations, seems to have changed that atmosphere considerably.